1. Field of the Invention
The present invention relates to optical discs and more particularly to methods and apparatus for duplicating data recordings, such as compact discs, as are used for computer, audio and video recording.
The present invention also relates to the fabrication of micro-structure in general, and in particular to the mass production of micro-miniaturized devices for such diverse fields as biomedical and micromachinery.
2. Description of the Prior Art
Compact disc read only memory (CD-ROM) discs and drives are now ubiquitous in computer data storage, audio recording of musical artists and video. Given the universal demand for CD-ROM titles, the manufacturing of CD-ROM duplicates from masters have assumed very large volumes.
CD-ROM manufacturing is concerned both with duplication fidelity and manufacturing cost. The manufacturing processes used in CD-ROM production closely resemble those used in the fabrication of semiconductors, especially the methods used in mask making.
Mastering, as its name implies, is the process of creating the disc from which all others are produced. The first step in the manufacturing process is to create a master that can be used for replication. A laser can be used to burn pits and lands containing the data into a photoresist surface beginning at the center track and moving outward in a spiral pattern.
After checking a glass master for accuracy, conventional replication machinery makes a stamper. Different replication processes require slightly different stampers, but the function remains the same, embossing the data pattern on the mass-production disc. Through an injection molding process, a series of intermediate impressions are made that provide a generation of negative stampers that produce positive disc images. The family-tree-like structure of this part of the production cycle has given rise to names such as mother father and sons or daughters for the various disc generations.
Compact discs (CDs) are typically made from a polycarbonate plastic, which is a material that is less vulnerable to water absorption and heat than polymethylmethacrylate (PMMA), which is universally used in laminated videodiscs. Videodiscs comprise two slices of substrate sandwiched together, so they are more rigid than CDs. Manufacturers take precautions to prevent heat or water-absorption warping, e.g., by using some type of injection molding in which polycarbonate resin is heated and poured into molds that shape the discs. A stamper impresses data patterns into the cooling plastic, and the disc is then put in a vacuum chamber, where a reflective layer of aluminum is added and coated with a protective lacquer. Labels are silk-screened or printed on the lacquer side.
Injection molding has a number of advantages. Plants worldwide use the technique, and its idiosyncrasies are well known. Yields are typically low when a manufacturing plant first opens, and increase substantially as experience is gained. Injection molding's critics claim the process is messy and requires large capital investments in equipment and clean rooms. During molding, polycarbonate distortions can appear in the plastic that impair or deflect a laser reading light. Despite its shortcomings, a number of manufacturing plants operating today use this process.
Minnesota Mining and Manufacturing Company (3M), for example, uses a prior art photo-polymerization (2P) process in which precut polycarbonate precursor resin is inserted between a master and a base plate, and then embossed. This polycarbonate precursor sandwich filling is then cured with ultraviolet light. This replication method has the advantage of being quick, which comes partially by avoiding any heating/cooling of the plastic during production. Critics of the 2P process say yields remain low because improper curing or warpage causes many discs to be rejected.
DOCData of Venlo, The Netherlands, and COMDisc of Los Angeles use two quite different methods that attempt to produce fast, low-cost replication of compact discs by a continuous printing or embossing technique. Both systems have worked in a laboratory setting, but neither is currently available commercially. Although the techniques show promise and have attracted a great deal of attention, no major company has yet committed itself financially to either process.
Masters are original copies of data recordings that are produced from tapes or software provided by artists and programmers. Lasers and electron beams (E-beams) are used as exposure tools for a photoresist carried on glass and photoplates. Semiconductor photomasks are similarly prepared.
Stampers are sub-masters duplicated from masters. Electroplating and photopolymers are two common ways that gaps in resist images are filled to produce reverse-tone sub-master duplicates of the masters so that the ultimate copies manufactured are positives of the masters.
The prior art photo-polymerization (2P) process starts with monomers that are irradiated to form polymers. In data recording disk duplication, such a process requires expensive machinery for ultraviolet irradiation and pressurizing the monomer solutions.